Contraband is the rarest label in CS2, and it belongs to a single skin: the M4A4 | Howl, the only Contraband item in the item dataset. The skin shipped in the Huntsman Case in 2014. In June 2014 Valve received a DMCA notice saying the Howl artwork and the Howling Dawn sticker were not the original work of their Workshop contributors. In its post Respecting Intellectual Property, Valve said both items were replaced by an alternative designed by the CS:GO team, the Huntsman Case was revised to remove them, and existing copies "will never be produced again, and have been assigned the Contraband rarity".
The label is a gold or amber colour, different from every other tier.
Why is the M4A4 Howl so expensive?
No new copies can ever be made. The Howl sits in no case and no collection, so it does not drop, cannot be unboxed, and cannot come out of a trade-up contract. Every Howl in the game today already existed when the skin was pulled. Some have been deleted or sit in abandoned accounts, so the supply only falls. Demand stays high because the skin is famous and the M4A4 is a popular rifle.
StatTrak copies and Factory New copies are rarer still, and Howls with well placed stickers from old Major capsules can be worth many times a plain copy. Souvenir versions never existed, since the Howl came from a case rather than a map collection.
Is Contraband a real rarity tier or just a label?
It is a real tier in the game files, but it exists for one item and behaves differently from the others. The Howl does not fit the trade-up ladder as an output, and it is not in any drop pool. The Howling Dawn sticker was pulled and replaced in the same dispute, and Valve assigned the Contraband rarity to both replaced items, but the Howl is the only weapon skin that carries it. Treat Contraband as a footnote in the rarity system rather than a tier you can aim for.